1. Field
This patent specification relates generally to radiation detection using nanotechnology. More particularly, this patent specification relates to methods and devices for determining at least one radiation property using an array of nano-tips, such as carbon nano-tubes (CNT), nano-wires, other types of field emitter array (FEA) or other types of local electric field enhancement arrays.
2. Background
Gas proportional chamber technology comprises a variety of different devices ranging from single-wire proportional counters, multi-layered multi-wire proportional chamber telescopes and multi-wire drift chambers. Applications span from simple radiation measurements, to neutron detection and charged particle tracking. They are used in almost every fundamental nuclear and particle physics research experiments, and widely used in industrial applications for radiation measurements.
For example, a gas proportional chamber filled with 3He gas is an efficient neutron detector due to the high neutron capture cross section of 3He. The typical proportional chamber consists of a hollow metallic cylinder that serves as the cathode, and a thin anode wire in the centre. The reaction products—a proton and a triton—are fast charged particles that ionize the 3He gas to produce electrons and ions. The electrons drift toward the anode wire under the influence of a guiding electric field which varies inversely with the radial distance from the wire. If the guiding electric field is above a critical value, the electrons will generate a small avalanche before reaching the anode wire. The critical value is usually achieved close to the wire due to the high electrical field in the vicinity of the thin wire. The avalanche amplifies the signal of the primary electron ion pairs generated by the charged reaction products. The size of the avalanche and the overall amplification depend on the electric field near the wire and the gas pressure. The avalanche process is illustrated in FIG. 1A, 1B, 1C and 1D. The gas pressure can be quite high in order to increase both the neutron detection efficiency and the stopping power for the reaction products before they reach the side walls. However, high gas pressure leads to slower electron and ion drifting velocities and limits the high counting rate capability of the device. The high pressure may also lead to other undesirable side effects such as high electron and ion recombination and neutron scattering, which may result in the neutron sensitivity not being a linear function of the pressure.
Another disadvantage to using 3He in a neutron counting tube is the supply-and-demand issue. 3He is becoming more expensive due to a huge increase in demand combined with a reduced supply. Because of those issues, alternative neutron detector technologies that do not require 3He gas are desirable. However, many proposed approaches have limitations for downhole applications in terms of implementation, efficiency, and measurement response. It should also be noted that a desirable solution must address the needs of existing tools in terms of detector space available and detector response.
Conventional gas chambers for example cannot be easily used for oil and gas applications, since the thin wires used in the gas chambers typically have a tiny diameter and have a large length ranging up to several meters depending on the applications and are normally stressed with a tension resulting in a very fragile and easily breakable structure. Further, the gas chamber of this type requires extreme care in construction and handling. Further still, the read out signals are prone to micro-phonic noises due to vibrations.
An alternative approach may include gas electron multipliers (GEMs) to using thin wires which require precision and tedious micro-machining and high voltages in operation, and are known for non-uniform efficiency and high-voltage break-downs (for example see Fabio Sauli, Leszek Ropelewski et al., the Gas Detectors Development Group at CERN, Geneva, Switzerland (http://gdd.web.cern.ch/GDD/)).
Therefore, there is a need for a detector that can handle very high instantaneous count rates, and still give a signal with the required signal to noise ratio.